1. Field of the Invention
This invention relates to wireless communications systems, and more particularly to cellular communication systems.
2. Description of Related Art
As noted in the application incorporated by reference above, in cellular communication systems, geographic areas or regions are typically divided into cells that are nominally hexagonally shaped. Each cell is allocated one or more radio frequency channels. In a frequency division multiple access (FDMA) system, adjacent or nearby cells are assigned separate frequencies. (The techniques described herein are primarily intended for use in FDMA systems, but may be used in time division multiple access (TDMA) or code division multiple access (CDMA) systems as well). After all available frequencies have been allocated, it is necessary to begin reusing the frequencies. For example, if seven frequencies are available, it is necessary to begin using the first frequency again starting in the eighth cell.
FIG. I is a block diagram of a prior art cell configuration showing a problem of frequency reuse Clusters of seven cells (modeled as hexagons for ease of understanding) form cell groups 1, indicated by bold lines. Seven frequencies (F.sub.1 to F.sub.7) are used within each cell group 1, and then reused in adjacent cell groups 1. Within each cell group 1, the pattern of frequency distribution is normally the same. Thus, the center cell of the central cell group shown uses the same frequency (F.sub.7) as the center cell of adjacent cell groups (F.sub.7).
Because frequencies are reused, two cells operating on the same frequency, though separated geographically, may interfere with each other. This is known as "co-channel interference". The effect of co-channel interference varies with terrain and distance. In cases where path loss conditions favor the desired signal, the co-channel interference may not be strong enough to have a significant impact on receiver performance. In other cases, path loss conditions may cause the difference between the desired carrier power and the interference (known as the "C/I" ratio) to be insufficient for good receiver performance. In many systems this occurs when the C/I ratio is below about 16-17 dB (generally indicative of significant co-channel interference), although the acceptable C/I ratio may be more or less, depending on the nature of the signal and the channel. The overall effect is to create areas within a cell where no good coverage is possible. In a case of seven total frequencies, these bad locations may comprise 40% or more of a typical cell.
The traditional way to mitigate co-channel interference in FDMA systems is to allocate a larger number of frequencies to the service and to devise sparse reuse patterns. A common allocation is a reuse factor of 21 (7 cells with three 120.degree. sectors per cell). However, this method cannot be used when only a small number of frequencies, such as seven, are available. The application incorporated by reference teaches time sharing of limited frequencies to reduce co-channel interference.
Although this technique reduces co-channel interference from cells in the same system, it does not reduce interference from other systems. Radio frequency spectrum is tightly allocated and, thus systems having closely allocated frequency spectrum may produce interfering transmissions. In addition, other devices, such as transformers, may produce electromagnetic energy whose frequency overlaps one or more frequencies allocated to a system. It is not uncommon for non-system radio sources to generate interference sufficient to prevent communication. As the technology which enables wireless communication has become more advanced and less expensive, systems that utilize radio communication have proliferated and continue to expand in number and size. This proliferation of systems increases the probability of interference between systems. If an interfering signal occurs on one or more frequencies (F.sub.1 to F.sub.7) in the coverage area of a cell shown in FIG. 1, then the interfering signal could create a region in a cell where communication by users within the cell may not be possible.
Accordingly, it is desirable to limit the interference within a cell from other radio systems or non-radio system sources. The present invention provides a solution to this problem.